Process for desulphurising and dewaxing a hydrocarbon feedstock boiling in the gasoil boiling range

ABSTRACT

A process for desulphurising and dewaxing a hydrocarbon feedstock boiling in the gasoil boiling range to obtain a desulphurised and dewaxed gasoil fraction for use in an ultra low sulphur diesel fuel, comprising the following steps: (a) hydrodesulphurising the hydrocarbon feedstock boiling in the gasoil boiling range by contacting the feedstock in a hydrodesulphurisation reaction zone at elevated temperature and pressure, in the presence of hydrogen, with a hydrodesulphurisation catalyst and withdrawing a liquid hydrodesulphurisation effluent from the reaction zone; (b) catalytically dewaxing the liquid hydrodesulphurisation effluent to obtain a desulphurised and dewaxed gasoil fraction by contacting the liquid hydrodesulphurisation effluent, at elevated temperature and pressure in the presence of hydrogen, with a stacked bed of catalysts having a first upstream bed of noble metal containing hydrofinishing catalyst and a second downstream bed of noble metal containing dewaxing catalyst, wherein hydrogen and the liquid effluent are countercurrently supplied to the stacked bed of catalysts.

FIELD OF THE INVENTION

The present invention provides a process for desulphurising and dewaxing a hydrocarbon feedstock boiling in the gasoil boiling range.

BACKGROUND OF THE INVENTION

It is known to produce ultra low sulphur diesel fuels by first hydrodesulphurising a hydrocarbon distillate stream boiling in the gasoil boiling range and then catalytically dewaxing the desulphurised distillate stream. The catalytic dewaxing step is needed for removing waxy molecules from the distillate stream in order to reduce the cloud point and the pour point of the gasoil. The desulphurised and dewaxed gasoil may be hydrofinished for saturation of aromatic compounds. The resulting desulphurised, dewaxed and optionally hydrofinished gasoil is then used as diesel fuel or diesel fuel component.

Also in the manufacture of lubricating oils, a dewaxing step is carried out for reducing the pour point and cloud point of the resulting lubricating oil.

In WO 98/02503, a process for producing a lubricating oil base stock has been described comprising catalytic dewaxing of a distillate fraction (138, 238) of a hydrocracker effluent (124, 224) in a stacked catalyst bed (140, 240). The upstream catalyst layer in the stacked bed is a hydrotreating catalyst and the downstream layer is a dewaxing catalyst. After the stacked bed dewaxing step (140, 240), a hydrofinishing step (160, 260) follows.

In EP 938 532 is disclosed a process for catalytically dewaxing a lube hydrocarbon feed containing less than 50 ppmw nitrogen employing a synergistic catalyst system having a hydrotreating catalyst preceding a dewaxing catalyst.

SUMMARY OF THE INVENTION

It has now been found that in a process for the preparation of ultra low sulphur gasoil for diesel fuels, it is advantageous to dewax a hydrodesulphurised gasoil in a stacked bed of catalysts wherein the upstream bed is a hydrofinishing catalyst and the downstream bed is a dewaxing catalyst whilst hydrogen and gasoil are countercurrently supplied to the stacked bed of catalysts. Reference herein to upstream and downstream is with respect to the flow direction of the liquid stream, i.e. the gasoil.

Accordingly, the present invention provides a process for desulphurising and dewaxing a hydrocarbon feedstock boiling in the gasoil boiling range to obtain a desulphurised and dewaxed gasoil fraction for use in an ultra low sulphur diesel fuel, comprising the following steps:

-   (a) hydrodesulphurising the hydrocarbon feedstock boiling in the     gasoil boiling range by contacting the feedstock in a     hydrodesulphurisation reaction zone at elevated temperature and     pressure, in the presence of hydrogen, with a hydrodesulphurisation     catalyst and withdrawing a liquid hydrodesulphurisation effluent     from the reaction zone; -   (b) catalytically dewaxing the liquid hydrodesulphurisation effluent     to obtain a desulphurised and dewaxed gasoil fraction by contacting     the liquid hydrodesulphurisation effluent, at elevated temperature     and pressure in the presence of hydrogen, with a stacked bed of     catalysts having a first upstream bed of noble metal containing     hydrofinishing catalyst and a second downstream bed of noble metal     containing dewaxing catalyst, wherein hydrogen and the liquid     effluent are countercurrently supplied to the stacked bed of     catalysts.

An advantage of the process according to the invention is that the dewaxing catalyst has an improved activity as compared to a process wherein the dewaxing step is carried out in co-current mode. Moreover, a higher sulphur concentration in the liquid hydrodesulphurisation effluent can be tolerated as compared to a similar stacked bed catalytic dewaxing process in co-current mode. In counter-current mode, the small amounts of hydrogen sulphide and ammonia that are formed in the first bed of hydrofinishing catalyst are removed from the stacked bed with the counter current stream of hydrogen and will not be contacted with the dewaxing catalyst in the second bed.

A further advantage of the process according to the invention is that no hydrofinishing step is needed downstream of dewaxing step (b).

A still further advantage is that the process equipment (hardware including catalysts) needed for the process according to the invention, which process can be considered as a so-called winter mode operation, is also suitable to operate in a so-called summer mode.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically depicts a flow diagram of an embodiment of the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The process according to the invention is a process for desulphurising and dewaxing a hydrocarbon feedstock boiling in the gasoil boiling range. The process comprises a hydrodesulphurisation step wherein a hydrocarbon feedstock boiling in the gasoil boiling range is contacted with a hydrodesulphurisation catalyst at hydrodesulphurisation conditions and a catalytic dewaxing step wherein hydrogen and the liquid hydrodesulphurisation effluent are countercurrently contacted with a stacked bed of hydrofinishing catalyst and dewaxing catalyst under hydrodewaxing conditions.

The feedstock may be any hydrocarbon stream substantially boiling in the gasoil boiling range, i.e. typically in the range of from 170 to 450° C. Typical examples of such feedstocks are straight-run gasoil, hydrocracked gasoil, thermal cracked gasoil, coker gasoil, vacuum gasoil, light or heavy cycle oil, or a combination of two or more thereof. Such feedstocks typically comprise sulphur-containing compounds, usually in a concentration in the range of from a few hundreds of ppmws to a few percent of sulphur. Reference herein to gasoil or to a hydrocarbon stream boiling in the gasoil boiling range is to a hydrocarbon stream of which at least 90 wt %, preferably at least 95 wt %, is boiling in the gasoil boiling range, i.e. in the range of from 170 to 450° C.

The hydrodesulphurisation catalyst may be any hydrodesulphurisation catalyst known in the art. Typically, these catalysts comprise a Group VIII metal and a compound of a Group VIB metal as hydrogenation components on a porous catalyst support, usually alumina or amorphous silica-alumina. Well-known examples of suitable combinations of hydrogenation compounds are cobalt-molybdenum, nickel-molybdenum, nickel-tungsten, and nickel-cobalt-molybdenum. A hydrodesulphurisation catalyst comprising compounds of cobalt and molybdenum as hydrogenation compounds is preferred. The hydrodesulphurisation catalyst may further comprise a cracking component such as for example Y zeolite. It is, however, preferred that no substantial hydrocracking takes place in hydrodesulphurisation step (a) of the process according to the invention. Therefore, it is preferred that the catalyst is substantially free of a cracking component. A catalyst comprising cobalt and molybdenum supported on alumina without a zeolitic cracking compound is particularly preferred.

The hydrodesulphurisation conditions in step (a), i.e. temperature, pressure, hydrogen supply rate, weight hourly velocity of the feedstock, are typical hydrodesulphurisation conditions. Preferably, the temperature in the hydrodesulphurisation step is in the range of from 300 to 400° C., more preferably in the range of from 320 to 380° C. Preferred hydrodesulphurisation pressures are in the range of from 20 to 80 bar (absolute).

It will be appreciated that the exact hydrodesulphurisation conditions will inter alia depend on the catalyst used, the sulphur content of the feedstock, the desired conversion of sulphur- and nitrogen-containing compounds and the extent to which hydrocracking of hydrocarbons boiling above 370° C. is allowed. Preferably, at most 10 vol % of the hydrocarbons boiling above 370° C. is hydrocracked into lower boiling compounds. Preferably, the liquid hydrodesulphurisation effluent has a sulphur content of at most 150 ppmw, more preferably at most 40 ppmw, even more preferably at most 20 ppmw, still more preferably at most 10 ppmw, particular preferably at most 5 ppmw. The nitrogen content of the liquid hydrodesulphurisation effluent is preferably at most 50 ppmw, more preferably at most 10 ppmw, even more preferably at most 5 ppmw.

It is within the normal skills of the skilled person to choose the hydrodesulphurisation conditions such that the desired sulphur and nitrogen conversion is obtained.

In the hydrodesulphurisation step, the greater part of the sulphur- and nitrogen-containing compounds in the feedstock are converted into hydrogen sulphide and ammonia, respectively. In the hydrodesulphurisation step, hydrogen and the feedstock may be supplied co-currently or counter-currently to the hydrodesulphurisation reaction zone, preferably co-currently. It will be appreciated that if hydrogen and liquid feedstock are co-currently supplied to the reaction zone, a vapour-liquid mixture is obtained as hydrodesulphurisation effluent. In that case, the effluent is separated into a liquid and a vapour effluent. Such vapour-liquid separation is common in hydroprocessing. The separation may be done by any method known in the art, for example by using a vapour/liquid separator such as a liquid draw-off tray, by stripping in a separator-stripper or by vapour/liquid separation followed by stripping of the thus-obtained liquid phase for removal of the dissolved hydrogen sulphide and ammonia. It will be appreciated that if the hydrodesulphurisation step is carried out counter-currently, a vapour effluent is withdrawn from the top of the hydrodesulphurisation reaction zone and a liquid effluent from the bottom of the reaction zone. In that case the liquid effluent as withdrawn from the hydrodesulphurisation reaction zone may be directly contacted with the stacked bed of dewaxing catalysts in step (b). Optionally, dissolved gases are removed from the liquid effluent, typically by stripping, before the liquid effluent is contacted with the stacked bed of dewaxing catalysts in step (b).

In step (b) of the process according to the invention, the liquid hydrodesulphurisation effluent is subjected to catalytic dewaxing by contacting it at catalytic hydrodewaxing conditions, i.e. at elevated temperature and pressure and in the presence of hydrogen, with a stacked bed of catalysts to obtain a desulphurised and dewaxed gasoil fraction. Hydrogen is supplied to the stacked bed of catalysts countercurrently with respect to the liquid hydrodesulphurisation effluent. The stacked bed of catalysts comprises a first, upstream bed of noble metal containing hydrofinishing catalyst and a second, downstream bed of noble metal containing dewaxing catalyst.

The stacked bed of catalysts may consist of a single bed of the first catalyst on top of a single bed of the second catalyst, i.e. without space between the first and the second bed. Alternatively, the first and the second bed may be spaced apart. Each of the first bed and the second bed may be divided in separate beds in series. In the case of two or more spaced-apart beds, interbed cooling is possible to remove heat released during the exothermic hydrofinishing or hydrodewaxing reaction, for example by means of an interbed quench.

The volume of the bed of hydrofinishing catalyst is preferably smaller than the volume of the bed of dewaxing catalyst. More preferably, the volume of the bed of hydrofinishing catalyst is in the range of from 10 to 65% of the volume of the bed of dewaxing catalyst. Reference herein to the volume of the bed of hydrofinishing catalyst is, in case of more than one bed of hydrofinishing catalyst, to the total volume of those beds without the interbed space. The same applies mutates mutandis for the volume of the bed of dewaxing catalyst.

In the process according to the invention, the whole liquid effluent of the first catalyst bed is supplied to the second catalyst bed.

The catalyst of the first bed is a hydrofinishing catalyst, i.e. a catalyst for aromatics saturation. The hydrofinishing catalyst may be any noble metal containing hydrofinishing catalyst known in the art. Hydrofinishing catalysts comprising palladium and/or platinum as hydrogenation component on a catalyst support comprising an amorphous refractory oxide, preferably amorphous silica-alumina or alumina, are particularly suitable.

The catalyst of the second bed is a noble metal-containing dewaxing catalyst. Any known dewaxing catalyst having a noble metal hydrogenation component may be used. Catalysts comprising a noble metal hydrogenation component, a dealuminated aluminosilicate zeolite and a refractory oxide binder material are particularly suitable. Such catalysts are for example disclosed in EP 1 137 740. The noble metal hydrogenation component is preferably platinum. Preferably, the catalysts comprises a dealuminated aluminosilicate zeolite having a Constraint Index of between 2 and 12, more preferably the zeolite is of the MTW type, even more preferably ZSM-12. The binder is preferably a non-acidic binder, more preferably silica.

The process conditions in step (b) of the process according to the invention, i.e. in the catalytic dewaxing step, are typical catalytic dewaxing conditions. Therefore, the temperature is typically in the range of from 250 to 400° C., the pressure typically in the range of from 20 to 80 bar (absolute). Hydrogen is typically supplied to the stacked catalyst bed at a rate of 350 to 750 Nl/kg gasoil.

Preferably, the temperature in the first catalyst bed is the same as in the second catalyst bed. It is, however, possible to operate the second catalyst bed at a slightly lower temperature, for example 10 to 20° C., than the first catalyst bed by cooling the liquid effluent from the first bed by using an interbed quench. This might be advantageous in case only limited dewaxing is needed.

It has been found that in the process according to the invention a very good gasoil yield is obtained if the temperature in the second catalyst bed is such that mild dewaxing of the gasoil, i.e. a cloud point reduction in the range of from 10 to 20° C., is achieved. Thus, it is preferred that the cloud point of the liquid effluent of step (b) is in the range of from 10 to 20° C. lower than the cloud point of the liquid effluent of step (a). This will typically be achieved at an operating temperature of the second bed in the range of from 300 to 340° C. An operating temperature of the second bed in the range of from 315 to 335° C. is particularly preferred.

It is an advantage of the process according to the invention that no hydrofinishing step for aromatics saturation and product stabilisation is needed after dewaxing step (b). The desulphurised and dewaxing gasoil fraction obtained in step (b) is particularly suitable to be sent to a diesel fuel blending pool directly, i.e. without further treatment. Reference herein to treatment is to a treatment wherein the molecular structure of gasoil components is changed and thus excludes blending.

In the process according to the invention, a desulphurised and dewaxed gasoil is obtained which is very suitable to be used as diesel fuel in cold environments, e.g. in winter time. In summer time, it will not always be necessary to reduce the pour point and cloud point of hydrodesulphurised gasoil, but it might be desirable to hydrogenate the gasoil for aromatics saturation or cetane or density improvement. It is an advantage of the process according to the invention that the equipment (hardware including catalysts) needed for the process can also be used to operate in a so-called summer mode. This so-called summer mode operation is similar to the process according to the invention, with the exception that the second bed of the stacked catalyst bed, i.e. the bed of noble metal containing dewaxing catalyst, is kept at low temperature, i.e. a temperature at which no dewaxing takes place. This may for example be achieved by quenching the liquid effluent of the first catalyst bed, i.e. the bed of hydrofinishing catalyst. In this way, the liquid hydrodesulphurisation effluent obtained in step (a) will only be hydrofinished and not be dewaxed. Thus, a gasoil suitable to be sent to a diesel fuel blending pool for summer-grade diesel fuel is obtained. It is noted that the summer-mode operation described above is not a process according to the invention.

DETAILED DESCRIPTION OF THE DRAWING

Referring now to FIG. 1, schematically showing a flow diagram of the process of the invention, the invention is further illustrated. A hydrocarbon feedstock 1 boiling in the gasoil boiling range and a hydrogen-rich gaseous stream 2 are fed to hydrodesulphurisation reaction zone 3. Hydrodesulphurisation reaction zone 3 contains a bed 4 of hydrodesulphurisation catalyst. In reaction zone 3, gasoil 1 and hydrogen-rich gaseous stream 2 are contacted with the catalyst in bed 4 at hydrodesulphurisation process conditions. The total effluent 5 of reaction zone 3 is separated in separator-stripper 6 into a gaseous effluent 7 and a liquid hydrodesulphurisation effluent 8. The gaseous effluent comprises hydrogen, C₄ ⁻ hydrocarbons and the hydrogen sulphide and ammonia formed. The liquid effluent 8 is supplied to catalytic dewaxing zone 10. A hydrogen-rich gaseous stream 9 is counter-currently supplied to catalytic dewaxing zone 10. Catalytic dewaxing zone 10 comprises, in a stacked bed configuration, a first upstream bed 11 of hydrofinishing catalyst and a second downstream bed 12 of dewaxing catalyst. In catalytic dewaxing zone 10, hydrocarbon stream 8 is first hydrofinished and then dewaxed at catalytic dewaxing process conditions. A desulphurised and dewaxed liquid effluent 13 is withdrawn from the bottom of reaction zone 10, a gaseous effluent 14 is withdrawn from the top of zone 10. A desulphurised and dewaxed gasoil fraction suitable for use in ultra low sulphur diesel fuel may be obtained by fractionating (not shown) liquid effluent 13. Preferably, liquid effluent 13 is fractionated together with the condensed gaseous effluent.

EXAMPLE

The process according to the invention is further illustrated by the following, non-limiting example.

Example Catalytic Dewaxing (Step (b))

A liquid hydrodesulphurisation effluent boiling in the gasoil boiling range having the properties as listed in Table 1 was contacted at a feed rate of 487 grams/h and a weight hourly space velocity of 3.1 kg oil/litre catalyst/hour with a stacked bed of catalysts comprising a layer of a commercially-available hydrofinishing catalyst (C-622; ex. Criterion Catalysts & Technologies) superimposed on a layer of hydrodewaxing catalyst (20 vol % hydrofinishing catalyst and 80 vol % hydrodewaxing catalyst). The hydrofinishing catalyst comprised Pt/Pd supported on amorphous silica-alumina; the hydrodewaxing catalyst comprised 0.7 wt % Pt on 30 wt % dealuminated ZSM-12 and 70 wt % silica.

The stacked bed was operated at different temperature in the range of from 322 to 340° C., at an outlet pressure of 60 bar (absolute). Hydrogen was supplied counter-currently to the stacked bed at a once through hydrogen gas rate of 500 Nl/kg oil.

In Table 2 are shown, the activity, yield and product properties for the experiment at which a cloud point reduction of 12° C. was achieved.

Compared to a similar test in co-current (same stacked bed of catalyst and same process conditions), it was found that for 12° C. cloud point reduction, a higher catalyst activity was achieved at a comparable gasoil yield. Reference herein to gasoil yield is to the yield of hydrocarbons boiling above 180° C. from both the liquid effluent (bottoms) and the condensed gaseous effluent (tops) of the dewaxing step. TABLE 1 Properties of liquid hydrodesulphurisation effluent cloud point (° C.) +1 sulphur content (ppmw) 140 nitrogen content (ppmw) <1 Monoaromatics (mmol/100 g) 91.0 diaromatics (mmol/100 g) 10.6 tri+ aromatics (mmol/100 g) 1.5 boiling point range (° C.) 0.5 wt % recovery (IBP) 80 5 wt % recovery 214 10 wt % recovery 237 20 wt % recovery 269 40 wt % recovery 303 60 wt % recovery 326 80 wt % recovery 354 90 wt % recovery 373 95 wt % recovery 389 99.5 wt % recovery (FBP) 435

TABLE 2 Results of catalytic dewaxing Activity (temperature¹ needed for 12° C. 331° C. cloud point reduction²) Yield (wt % on feed) gas make 0.8 total liquid product³ (>80° C.) 98.5 gasoil⁴ (>180° C.) 93.6 hydrogen consumption (wt % on feed) 0.4 Composition of total liquid product sulphur content (ppmw) 2 nitrogen content (ppmw) <1 ¹weighted average bed temperature of total stacked bed. ²cloud point of total liquid product³ as compared with with cloud point of feed. ³‘total liquid product’ are the hydrocarbons boiling above 80° C. from both the liquid effluent (bottoms) and the condensed gaseous effluent (tops) of the dewaxing step. ⁴hydrocarbons boiling above 180° C. from both the liquid effluent (bottoms) and the condensed gaseous effluent (tops) of the dewaxing step. 

1. A process for desulphurising and dewaxing a hydrocarbon feedstock boiling in the gasoil boiling range to obtain a desulphurised and dewaxed gasoil fraction for use in an ultra low sulphur diesel fuel, comprising the following steps: (a) hydrodesulphurising the hydrocarbon feedstock boiling in the gasoil boiling range by contacting the feedstock in a hydrodesulphurisation reaction zone at elevated temperature and pressure, in the presence of hydrogen, with a hydrodesulphurisation catalyst and withdrawing a liquid hydrodesulphurisation effluent from the reaction zone; (b) catalytically dewaxing the liquid hydrodesulphurisation effluent to obtain a desulphurised and dewaxed gasoil fraction by contacting the liquid hydrodesulphurisation effluent, at elevated temperature and pressure in the presence of hydrogen, with a stacked bed of catalysts having a first upstream bed of noble metal containing a hydrofinishing catalyst and a second downstream bed of noble metal containing a dewaxing catalyst, wherein hydrogen and the liquid effluent are countercurrently supplied to the stacked bed of catalysts.
 2. A process according to claim 1, wherein the volume of said first upstream bed of hydrofinishing catalyst is smaller than the volume of said second downstream bed of dewaxing catalyst.
 3. A process according to claim 2, wherein said hydrodesulphurisation catalyst comprises a cobalt compound and a molybdenum compound as hydrogenation compounds.
 4. A process according to claim 3, wherein said hydrocarbon feedstock is contacted with the hydrodesulphurisation catalyst in step (a) at a temperature in the range of from 300 to 400° C., and at a pressure in the range of from 20 to 80 bar (absolute).
 5. A process according to claim 4, wherein the dewaxing catalyst comprises a noble metal as hydrogenation component, a dealuminated silica-alumina zeolite and a refractory oxide binder material.
 6. A process according to claim 5, wherein the dealuminated silica-alumina zeolite is dealuminated ZSM-12.
 7. A process according to claim 6, wherein the temperature in step (b) is in the range of from 250 to 400° C.
 8. A process according to claim 7, wherein the temperature in the second downstream bed is in the range of from 300 to 340° C.
 9. A process according to claim 8, wherein the pressure in step (b) is in the range of from 20 to 80 bar (absolute).
 10. A process according to claim 9, wherein the liquid hydrodesulphurisation effluent has a sulphur content of at most 150 ppmw.
 11. A process according to claim 10, wherein the liquid hydrodesulphurised effluent has a nitrogen content of at most 50 ppmw.
 12. A process according to claim 11, wherein the desulphurised and dewaxed gasoil fraction obtained in step (b) is not subjected to a hydrofinishing step before being used in a ultra low sulphur diesel fuel. 